Historically, wind turbines have been very small contributors to overall power generation to supply electrical grids. The low unit ratings (<100 kW) and the uncertain availability of wind sources caused wind turbine generators to be ignored when power grid operators considered the security of the grid. However, wind turbine generators with ratings of 1.5 MW or more are now available. Furthermore, many power generation developers are installing wind farms having one hundred or more wind turbine generators. The “block” of power available from wind farms with 1.5 MW wind turbine generators is comparable to a modern gas turbine generator. Accordingly, wind turbine generators are increasingly feasible sources of power for the power grid.
In order to reliably supply power to the power grid, wind turbine generators (as well as other types of generators) must conform to power grid interconnection standards that define requirements imposed on power suppliers and large power consumers. In particular, a “low voltage ride through” (LVRT) requirement typically requires that a power generation unit must remain connected and synchronized to the grid when the voltage at the terminals of the generation unit fall to prescribed levels.
The LVRT requirement has been addressed in steam and gas turbine generator plants through use of vital electrical buses that are powered by DC power sources and by auxiliary buses connected to the generators. These types of generations are generally more resistant to voltage fluctuations than wind turbine generators.
In the past, wind turbine generators have been allowed to trip offline during a low voltage event. For example, the most common safety concept of wind turbine generators is a battery buffered pitch system, which typically includes three independent battery packs. With this type of system it is possible to turn the blades of the wind turbine from an operating position to a park position when generator power is not available.
During a power failure, the pitch drives are switched from a generator powered drive to a battery powered drive until the blades reach the park position. The park position is typically defined by an end limit switch that disconnects the motor from the batteries. The movement of the blades to the park position occurs automatically as the result of a voltage or frequency error. However, this does not satisfy LVRT requirements because the wind turbine generator is allowed to trip offline.
Currently, wind turbine generators specifications can require connection and synchronization with the power grid down to levels of 70% of rated voltage. These requirements can be accommodated through, for example, increased capacity in various components (motors, generators, converters, etc.) and by use of uninterruptible power supplies (UPSs) for sensitive control circuits. However, more severe voltage fluctuations, for example, voltages at 15% of rated voltage cannot be accommodated using these techniques.